Communications equipment is often sensitive to the loss of mains power. In some systems, either a standby source of mains power is provided, or a method of detecting the loss of mains power is employed in conjunction with an energy storage element which provides enough energy for the powered equipment to complete any tasks that must be finished before the energy storage element is depleted.
One class of system extensively discussed in the prior art is one which senses a power fail event, and communicates this to a processor or system. U.S. Pat. No. 4,509,201 by Sekigawa et al discloses a voltage monitor coupled to a battery which signals when the battery has reduced operating level, thereby producing a signal to a CPU. U.S. Pat. No. 4,509,874 by Shimamoto et al. includes a variable A/D converter which measures the output voltage of two batteries and an AC/DC converter for use by a CPU. U.S. Pat. No. 5,831,805 by Sekine discloses a power failure detection subsystem for determining the loss of power and sending this information to a CPU.
A second class of system uses this loss of power signal to start a process related to saving information from a pre-existing process. U.S. Pat. No. 5,283,792 by Davies et al, U.S. Pat. No. 5,339,446 by Yamasaki et al, and U.S. Pat. No. 5,423,045 by Kannan et al. disclose the storage of CPU data and instructions upon the detection of power failure or imminent loss of power.
A third class of system uses a local capacitor to store sufficient charge to enable a system to operate on the content of this charge alone. U.S. Pat. No. 5,553,138 by Heald et al discloses a CPU which derives its power from a telephone hook voltage, and senses when the level of voltage on this storage capacitor is diminished, thereby producing a signal.